The poly(ethenylidene-co-2,5-tetrahydrofuran) materials resulting from the polymerization of 7-oxabicyclo[2.2.1]hept-5-ene (hereinafter, 7-oxanorbornene) monomers are of keen interest, due to their potential ionophoric properties. Molecular model studies indicate that these poly(7-oxanorbornene) polymers have the ability to form helical structures, with all of the tetrahydrofuran oxygen facing into the interior of the helix. This unique helical conformation may allow these polymers, when in solution, to act as useful acyclic ionophores, much like their cyclic analogues, the cyclic crown ethers. In addition, thin films composed of these poly(7-oxanorbornene) materials may posses oxygen-rich ionophoric channels that would enable them to act as ion permeable membranes.
Classical ROMP catalysts react with, and are deactivated by, the 1,4 bridging epoxide moiety present in the 7-oxanorbornane derivatives. For this reason, it is important to find catalysts capable of successfully polymerizing 7-oxanorbornene and its derivatives.
Attempts at the emulsion polymerization of various norbornene derivatives have been made; see R. E. Rinehart et al, "The Emulsion Polymerization of the Norbornene Ring System Catalyzed by Noble Metal Compounds", Polymer Letters, Vol. 3, pp. 1049-1052 (1965) and R. E. Rinehart, "Polymerizations Catalyzed by Noble Metal-Olefin Complexes", Journal of Polymer Science, Part C, No. 27, pp. 7-25 (1969). However, all of these polymerizations require the use of emulsifiers and various co-catalysts. When the polymerizations were attempted without these additives, no polymer was formed. Even with the added emulsifiers and added co-catalysts, the reported yields were typically less than 10%.
The benefits of emulsion polymerization systems are well documented. These benefits include ease of catalyst separation, good heat transfer during the reaction, ease of product manipulation, and kinetic parameters that allow for the production of high molecular weight materials. In addition, water is a desirable industrial solvent because of its cost, non-toxic nature and ease of disposal. All of the above reasons provide the impetus to find new catalytic systems that work well in water.
Thus, there remains a need for both a facile method of obtaining polymers of 7-oxanorbornene and its derivatives in high yields and the development of effective transition metal catalyzed emulsion polymerization systems.
Ring opening metathesis polymerization (ROMP) methods have been shown to be quite effective for the polymerization of strained cyclic, olefinic hydrocarbons. This technique has been expanded by the development of well-characterized alkylidene catalysts, which are able to produce living monodispersed polymers. These living polymers can be specifically end-capped with a variety of carbonyl compounds. The extension of ROMP methods, however, to monomers other than hydrocarbons has been significantly more challenging.
Metathesis polymerizations of monomers containing pendant functionalities have met with only limited success, and successful metathesis polymerizations of strained heterocyclic monomers are even more rare. These limitations are primarily the result of side reactions between the heteroatoms in the monomers and the typically oxophilic alkylidene ROMP catalysts.